Recent epidemiological studies have found an association between particulate air pollution and cardiopulmonary health effects, including increased asthma symptoms. There have been anecdotal reports of increased asthma symptoms associated with the use of oxygenated fuels, which are associated with increased emissions of aldehydes by motor vehicles. However, the biological mechanisms underlying these reported health effects are unknown. The purpose of this study is to test the hypotheses that indices of asthma are associated with ambient submicron particle counts and with ambient formaldehyde concentrations. This will be a prospective time-series study of the association between daily air pollutant concentrations and daily symptom scores, medication use, and changes in lung function in a cohort of 60 young adults in downtown Denver, Colorado. The association between changes in asthma indices and baseline bronchial hyper-responsiveness will also be determined. The results will help determine the size fraction of particles which are most highly associated with changes of asthma indices, and may support an osmotic mechanism to explain worsening of asthma associated with particulate air pollution. Similarly, the results will help to clarify whether ambient aldehyde concentrations are associated with worsening of asthma.